1. Field of the Invention
This invention relates to a data modulating method and apparatus and a data demodulating method and apparatus suitable for use for digitally recording data onto a record medium, and more particularly to a data modulating method and apparatus and a data demodulating method and apparatus wherein a modulated code in the form of a variable length code is used and record data in units of m bits are converted into a channel code in units of n bits.
2. Description of the Prior Art
In a magnetic recording system, generally a signal has a differentiation frequency characteristic and suffers from deterioration in a high frequency band. This arises from a loss at a head gap, a loss by a space between a head and a record medium, a loss originating in thickness of a record medium, a low frequency band loss at a rotary transformer and so forth. Further, noise such as crosstalk noise from an adjacent track, noise from a record medium and overwrite noise could cause random errors. In order to record and reproduce data accurately irrespective of such losses and noise, digital information should be recorded onto a record medium after it is modulated so that it may be compatible with a recording system, which allows a greater amount of information to be recorded stably. To this end, channel coding of data is performed in accordance with a predetermined rule.
One of such channel coding methods is block coding. The block coding involves dividing a data sequence into blocks of m.times.i bits and converting each data word into a channel code of n.times.i channel bits in accordance with a suitable coding rule. When i=1, the channel code is a fixed length code, but when i is greater than 1 and the binding length r is greater than 1, the channel code is a variable length code. The block code is also called (d, k;m, n;r) code. Here, d is a minimum run number of 0 while k is a maximum run number of 0.
Such block coding is disclosed in Japanese Patent Laid-Open Application No. Heisei 01-221919, laid open on Sep. 5, 1989, under the title of "Variable Length Code Converting Method". According to the prior art, the run numbers of 0 at the beginning and last ends of a code word are restricted and a decisively decodable code word is selected. Consequently, this code has a minimum magnetization reversal distance (Tmin) which is 1.33 times that of a conventional (2, 7) RLL code and is twice that of the MFM. In particular, according to the prior art, the minimum data length m is m=2, the minimum code word length n is n=6, the code word length number rmax is rmax=4, and the run number of "0" of a binary code word train of continuous code words after conversion is restricted to be inclusively between 5 and 16 (d, k). The requirements for such variable length code to be used as a code word include three requirements that the (d, k) restriction is satisfied within one code word, that the (d, k) restriction is satisfied when code words are connected to each other, and that a boundary of a code word can be discriminated correctly and can be decoded decisively, and an additional requirement that code words are allotted preferentially from that one which includes a greatest number of "1"s so that the average magnetization reversal distance may be minimized.
Similar block coding techniques are disclosed in Japanese Patent Laid-Open Applications Nos. Heisei 01-221920, Heisei 01-221921, Heisei 01-221922, Heisei 01-221923, and Heisei 01-221925, all laid open on Sep. 5, 1989.
The variable length codes disclosed in the prior art documents are disadvantageous in that, when compared with the variable length code employed in the present invention, the detecting window width Tw is narrower and the detecting efficiency upon decoding is lower.
Another block coding technique is disclosed in Japanese Patent Laid-Open Application No. Showa 59-160357, laid open on Mar. 3, 1983 under the title of "Coding and Decoding Methods for Binary Data", wherein binary data of 2 bits is converted into a conversion code of 5 bits. This technique, while making the most of a magnetization reversal distance of the HDM-3, enhances the detecting window width to 0.4 T to allow recording and reproduction of data at a high density.
More particularly, according to this block coding technique, binary data are outputted successively in synchronism with a clock signal to terminals for parallel outputs of a serial/parallel register of 10 bits. A coder receives, at input terminals thereof, the data from the output terminals of the serial/parallel register, generates a 5-bit conversion code in accordance with a coding algorithm, and outputs it to a 5-bit output terminal. The 5-bit conversion code is inputted to a 5-bit serial/parallel resistor, which outputs a conversion code train at an output terminal.
This coding method, however, does not involve a variable length code but involves a fixed length code, and employs a code resembling the variable length code employed in the present invention. However, the algorithm for generating the code is complicated, and accordingly, the hardware configuration is complicated.
A further block coding technique is disclosed in "Proposal of (3, 19;4, 9;3 ) Code", Collection of Theses of Spring National Conference of Electronic Information Communication Society, 1989, which describes a (3, 19;4, 9;3) code as a concrete example of a (3, k) variable length code. The code is superior in minimum magnetization reversal distance (Tmin) to the (2, 7) RLL code or the 2/3 code and is superior in Tmin.times.Tw (minimum magnetization reversal distance x detecting window width) to the (2, 7) RLL code.
It will also be noted that characteristic evaluation of the case wherein the code is to be used for a magneto-optic system is disclosed in Television Society Bulletin, Vol. 44, No. 10, pp. 1369-1375, 1990.
By the way, a fixed length code sometimes employs a dummy code in order to adjust the code length, and has a potential difficulty in that data cannot be recorded at a high density. In contrast, a variable length code does not use a dummy code, and accordingly, higher density recording is allowed. However, a variable length code has a potential difficulty in that a dc component cannot be controlled.